In an engine control system having an EGR valve, it is likely that an opening degree of the EGR valve may vary due to a differential pressure between an exhaust pressure and an intake pressure. Japanese patent No. 3494831 shows an engine control system in which a holding force for holding an opening degree of the EGR valve is established according to a differential pressure between an exhaust pressure and an intake pressure.
In an engine control system having a supercharger, when an internal combustion engine is at a low-rotation high-load operation state, the intake pressure exceeds the exhaust pressure for a certain time period in an exhaust stroke and an intake stroke. At this engine operation state, a valve overlap quantity between an intake valve and an exhaust valve is increased to increase the differential pressure between the intake pressure and the exhaust pressure, whereby a scavenging of cylinders is expedited. In this case, since the differential pressure is further increased, it is likely that a holding force for holding the EGR valve at full-close position may become insufficient, so that the EGR valve may be opened even if the controller commands the EGR valve to be closed. As a result, a fresh intake air may flow into an exhaust passage through the EGR valve, which deteriorates a supercharging performance of the turbocharger.
In view of Japanese patent No. 3494831, it is conceivable that the holding force for holding the EGR valve at full-close position is established according to a differential pressure between the intake pressure and the exhaust pressure. However, in this case, since the holding force is established after actually detecting the differential pressure between the intake pressure and the exhaust pressure, an increase of the holding force may delay relative to an increase of the differential pressure, so that the EGR valve may not be held at the full-close position.